1. Field of the Invention
The present invention relates to a power module used in a semiconductor device for power control, and more specifically, to a protection circuit of a semiconductor element mounted on the power module.
2. Description of the Background Art
A power module known to be used in a semiconductor device for power control has a protection function for protecting a semiconductor element such as an IGBT (insulated gate bipolar transistor) functioning as a switching element from overcurrent or a short circuit. Such a power module generally includes a resistor (sense resistor) for converting a current (sense current) flowing in a sense terminal of the semiconductor element to a voltage (sense voltage), and a protection circuit that performs predetermined operation for protection based on the sense voltage (see Japanese Patent Application Laid-Open Nos. 2005-348429 and 5-275999 (1993), for example). The protection circuit determines that a current flowing in the semiconductor element has exceeded an admissible value if the sense voltage has reached a predetermined value, and then performs operation for protection. As an example, the protection circuit stops the operation of the semiconductor element.
A power module having the conventional protection function generates fluctuations of the ratio of a sense current diverted from a main current or fluctuations of the resistance value of a sense resistor. This generates fluctuations of a current value that is what is called a “short-circuit protection trip level (SC trip level)” at which a protection circuit starts operation for protection, making it difficult to realize the operation for protection appropriately. Further, the upper limit of the current value of a semiconductor element should be changed in some cases according to the change of environment where the power module is used. The power module suggested in Japanese Patent Application Laid-Open Nos. 2005-348429 and 5-275999 that allows adjustment of the SC trip level of a protection circuit may be responsive to these problems.
In the conventional power module, if a power source voltage (control power source voltage) for controlling the semiconductor element drops for some reason, the voltage level of a drive signal for driving the semiconductor element is reduced. This causes the semiconductor element to operate easily in an active region (in the following, operation of a semiconductor element in an active region is called “active operation”). The active operation of the semiconductor element generates thermal breakdown easily due to increase of an ON voltage. If the semiconductor element is an IGBT, for example, the active operation of the IGBT increases an ON voltage (collector-to-emitter voltage) applied when a collector current flows. This generates serious thermal loss, leading to generation of thermal breakdown of the IGBT.
As already described above, in the conventional power module, an SC trip level fluctuates due to fluctuations of the ratio of a sense current diverted from a main current or fluctuations of the resistance value of a sense resistor. In particular, if the SC trip level of the power module shifts to a higher level and if the control power source voltage thereof drops, the semiconductor element performs active operation before a current flowing therein reaches the SC trip level, namely before the protection circuit works, generating a problem of thermal breakdown.